Flat top laser beam processing for making a solar cell substrate

ABSTRACT

Flat top beam laser processing schemes are disclosed for producing various types of hetero-junction and homo-junction solar cells. The methods include base and emitter contact opening, back surface field formation, selective doping, and metal ablation. Also, laser processing schemes are disclosed that are suitable for selective amorphous silicon ablation and selective doping for hetero-junction solar cells. These laser processing techniques may be applied to semiconductor substrates, including crystalline silicon substrates, and further including crystalline silicon substrates which are manufactured either through wire saw wafering methods or via epitaxial deposition processes, that are either planar or textured/three-dimensional. These techniques are highly suited to thin crystalline semiconductor, including thin crystalline silicon films.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/271,212 filed Oct. 11, 2011 claiming priority to U.S. ProvisionalPat. App. 61/391,863 filed Oct. 11, 2010, both of which are herebyincorporated by reference in its entirety.

This application is also a continuation-in-part of U.S. patentapplication Ser. No. 13/118,295 filed May 27, 2011, U.S. patentapplication Ser. No. 11/868,488 filed Oct. 6, 2007, U.S. patentapplication Ser. No. 11/868,492 filed 10 Oct. 6, 2007, U.S. patentapplication Ser. No. 12/774,713 filed May 5, 2010, and U.S. patentapplication Ser. No. 13/057,104 filed Feb. 1, 2011, which are alsohereby incorporated by reference in their entirety.

FIELD

This disclosure relates in general to the field of solar photovoltaics,and more particularly to laser processing techniques for the productionof crystalline semiconductor, including crystalline silicon, and othertypes of photovoltaic solar cells.

BACKGROUND

Laser processing offers several advantages in terms of efficiencyenhancement and manufacturing cost reduction for high-performance,high-efficiency solar cell processing. Firstly, advanced crystallinesilicon solar cells may benefit from having the dimensions of thecritical features such as electrical contacts be much smaller than thecurrent industrial practice. For front contacted solar cells the contactarea of the front metallization to the emitter as well as the contactarea of the back metal to the base needs to be low (or the contact arearatios should be fairly small, preferably much below 10%). For an allback-contact, back-junction solar cell, where the emitter and baseregions forming the p/n junction and the metallization are on the sameside (the cell backside opposite the sunny side), the dimensions of thevarious features are typically small for high efficiency. In these cellswhere typically the emitter and base regions form alternate stripes, thewidth of these regions (in particular the width of the base contact)tends to be small. Also, the dimensions of the metal contacts to theseregions tend to be proportionally small. The metallization connecting tothe emitter and base regions then needs to be patterned to acorrespondingly finer scale. Generally, lithography and laser processingare the technologies that have the relatively fine resolution capabilityto provide the small dimensions and the control required. Of thesetechniques, only laser processing offers the low cost advantage requiredin solar cell making. While lithography requires consumables such asphotoresist and subsequent resist developer and stripper (which add tothe process cost and complexity), laser processing is a non-contact,dry, direct write method and does not require any material consumables,making it a simpler and lower cost process for solar cell fabrication.Moreover, laser processing is an excellent choice for environmentallybenign manufacturing since it is an all-dry process which does not useany consumables such as chemicals.

Further, to reduce the cost of solar cells there is a push to reduce thethickness of the crystalline silicon used and also at the same timeincrease the cell area for more power per cell and lower manufacturingcost per watt. Laser processing is suitable for these thin wafers andthin-film cell substrates as it is a completely non-contact, dry processand can be easily scaled to larger cell sizes.

Laser processing is also attractive as it is generally a “green” andenvironmentally benign process, not requiring or using poisonouschemicals or gases. With suitable selection of the laser and theprocessing system, laser processing presents the possibility of veryhigh productivity with a very low cost of ownership.

Despite these advantages, the use of laser processing in crystallinesilicon solar cell making has been limited because laser processes thatprovide high performance cells have not been developed. Disclosed hereare laser processes using schemes that are tailored for each keyapplication to produce solar cells with high efficiency. Specificembodiments are also disclosed for applications of laser processing inmanufacturing thin-film crystalline silicon solar cells, such as thosemanufactured using sub-50-micron silicon substrates formed by epitaxialsilicon growth.

SUMMARY

Laser processing schemes are disclosed that meet the requirements ofbase to emitter isolation (including but not limited to shallow trenchisolation) for all back-contact homo-junction emitter solar cells (suchas high-efficiency back-contact crystalline silicon solar cells),opening for base doping, and base and emitter contact opening (withcontrolled small contact area ratios, for instance substantially below10% contact area ratio, for reduced contact recombination losses andincreased cell efficiency), selective doping (such as for base and/oremitter contact doping), and metal ablation (formation of patternedmetallization layers such as creating the patterned metallization seedlayer on a thin-film monocrystalline silicon solar cell prior tosubsequent attachment of a backplane to the cell and its release from areusable host 5 template) for both front-contact and allback-contact/back-junction homo-junction emitter solar cells. Also,laser processing schemes are disclosed that are suitable for selectiveamorphous silicon ablation and oxide (such as a transparent conductiveoxide) ablation, and metal ablation for metal patterning forhetero-junction solar cells (such as back-contact solar cells comprisinghetero-junction amorphous silicon emitter on monocrystalline siliconbase). These laser processing techniques may be applied to semiconductorsubstrates, including crystalline silicon substrates, and furtherincluding crystalline silicon substrates which are manufactured eitherthrough wire saw wafering methods or using epitaxial depositionprocesses, that are either planar or textured/three-dimensional, wherethe three-dimensional substrates may be obtained using epitaxial siliconlift-off techniques using porous silicon seed/release layers or othertypes of sacrificial release layers. These techniques are highly suitedto thin crystalline semiconductor, including thin crystalline siliconfilms obtained using epitaxial silicon deposition on a templatecomprising a porous silicon release layer or other techniques known inthe industry and can have any crystalline silicon absorber thickness inthe range of from less than one micron to more than 100 microns (withthe thin-film monocrystalline silicon solar cells preferably having asilicon thickness of less than 50 microns).

An all back-contact homo-junction solar cell is formed in thecrystalline silicon substrate, wherein laser processing is used toperform one or a combination of the following: micromachine or patternthe emitter and base regions including base to emitter isolation as wellas openings for base, provide selective doping of emitter and base, makeopenings to base and emitter for metal contacts, and provide metalpatterning. A front contacted homo-junction (emitter) solar cell may bemade using laser processing for selective doping of emitter and makingopenings for metal contacts for both frontside and backsidemetallization. A hetero-junction all back-contact back-contact solarcell may be made using laser processing for defining the base region andconductive oxide isolation.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, nature, and advantages of the disclosed subject matterwill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings, in which like referencenumerals indicate like features and wherein:

FIG. 1 shows a scanning electron microscope (SEM) image of a shallowtrench made in silicon for application in an all back contact solarcell, in accordance with the present disclosure;

FIG. 2 shows a profile of a shallow trench in silicon for application inall back contact solar cells;

FIGS. 3A-3D show the procedure for selecting the laser fluence to obtainreduced damage silicon dioxide (or oxide) ablation. FIG. 3A shows thedependence of the size of the ablation spot on the laser fluence; FIG.3B shows irregular delamination of oxide; FIG. 3C shows a damage-freespot; and FIG. 3D shows highly damaged silicon in the spot opening;

FIG. 4 shows substantially parallel rows of contacts opened in oxideusing pulsed laser ablation in accordance with the present disclosure;

FIG. 5 shows a screenshot with oxide ablation spots for metal contacts;

FIGS. 6A and 6B show the laser-ablated area formed by making ablationspots that are overlapped in both the x and y-direction; FIG. 6A shows a18 micron wide strip opened in 1000 A BSG (boron-doped oxide)/500 A USG(undoped oxide) for base isolation region; and FIG. 6B shows a 90 micronwide stripe opened in 1000 A USG (undoped oxide) for base region;

FIG. 7A shows the threshold for oxide damage, below which metal can beremoved without metal penetration of the oxide layer;

FIG. 7B shows that after 20 scans the metal runners are fully isolated;

FIG. 7C shows an optical micrograph of the trench formed in this metalstack;

FIGS. 8A and 8B show a top view and a cross-sectional view of apyramidal TFSC;

FIGS. 9A and 9B show a top view and a cross-sectional view of a prismTFSC;

FIGS. 10A and 10B show a process flow for creation and release of aplanar epitaxial thin film silicon solar cell substrate (TFSS);

FIGS. 11A and 11B show a process flow for planar epitaxial thin filmsilicon solar cell substrate in case the TFSS is too thin to be freestanding or self-supporting;

FIGS. 12A and 12B show a process flow for micromold template (orreusable template) creation for making a 3-D TFSS;

FIGS. 12C and 12D show a process flow for 3-D TFSS creation using thereusable micromold template;

FIG. 13 shows a process flow for making a planar front contacted solarcell where the TFSS is thick enough to be free standing andself-supporting (e.g. thicker than approximately 50 microns for smaller100 mm×100 mm substrates and thicker than approximately 80 microns for156 mm×156 mm substrates), in accordance with the present disclosure;

FIG. 14 shows a process flow for making a planar front contact solarcell where the TFSS is too thin to be self supporting, in accordancewith the present disclosure;

FIG. 15 shows a process flow for making a 3-D front contact solar cellin accordance with the present disclosure;

FIGS. 16A-16D show a process flow for making an interdigitated backcontact back-junction solar cell where the TFSS is thick enough to beself supporting, in accordance with the present disclosure;

FIG. 17 shows a process flow for making an interdigitated back-contactback-junction solar cell using thick TFSS where the in-situ emitter isnot deposited. Instead, a BSG (boron-doped oxide) layer is deposited onthe epitaxial silicon film and patterned to open the base isolationregion, in accordance with the present disclosure;

FIG. 18 shows a process flow for making an interdigitated back-contactback-junction solar cell where the TFSS is not thick enough to be selfsupporting, where in-situ emitter and laser ablation of silicon is usedto form the base isolation opening, in accordance with the presentdisclosure;

FIGS. 19A-19H show a process flow for making an interdigitatedback-contact back-junction solar cell where the TFSS is not thick enoughto be self supporting, and where instead of in-situ emitter BSG(boron-doped oxide) deposition and selective laser etchback is used toform the base isolation opening, in accordance with the presentdisclosure;

FIG. 20 shows a process flow for making an interdigitated back-contactback-junction solar cell using a 3-D TFSS, in accordance with thepresent disclosure;

FIG. 21 shows a process flow for making an interdigitated back-contactback-junction hetero-junction solar cell, in accordance with the presentdisclosure;

FIGS. 22A through 30 are not found in U.S. patent application Ser. No.13/118,295 “LASER PROCESSING FOR HIGH-EFFICIENCY THIN CRYSTALLINESILICON SOLAR CELL FABRICATION” by Virendra V. Rana and filed on May 27,2011;

FIGS. 22A and 22B are schematics showing the profile of a Gaussian beamand a flat top beam, respectively;

FIG. 23 is a cross-sectional diagram of a back-contact/back-junctioncell;

FIGS. 24A, 24B, 24C, 24D, 24E, and 24F are rear/backside views of a backcontact solar cell during fabrication;

FIG. 25 is a rear/backside view of the back contact solar cell of FIG.24A with alternating metal lines contacting the emitter and baseregions;

FIGS. 26A-26C are diagrams illustrating three ways a flat-top beamprofile may be created;

FIGS. 27A and 27B are schematics showing the profile of a Gaussian beamand a flat top beam highlighting the ablation threshold;

FIGS. 28A and 28B are diagrams showing a Gaussian beam and a flat topbeam ablate region profile/footprint, respectively;

FIG. 28C is a graph of overlap and scan speed;

FIGS. 29A and 29B are diagrams illustrating a beam alignment window of aGassian beam and flat top beam, respectively;

FIGS. 30A and 30B are diagrams showing a Gaussian beam region profileand a flat top beam region profile, respectively; and

FIG. 30C graphically depicts the results of Table 1.

DETAILED DESCRIPTION

Although the present disclosure is described with reference to specificembodiments, one skilled in the art could apply the principles discussedherein to other areas and/or embodiments without undue experimentation.

We present here laser processing, more specifically pulsed laserprocessing, schemes that have been developed to address the varyingrequirements of different processes.

The disclosed methods may be useful in the area of semiconductor deviceablation, particularly crystalline silicon ablation. Typically removalof silicon with a laser involves silicon melting and evaporation thatleaves undesirable residual damage in the silicon substrate. This damagecauses minority carrier lifetime degradation and increased surfacerecombination velocity (SRV) that reduces the solar cell efficiency.Hence, wet cleaning of the silicon substrate is typically used to removethis damage layer. We present a scheme to reduce this damage to a levelacceptable for high efficiency solar cell manufacturing that does notrequire post-laser-processing wet cleaning, hence simplifying theprocess flow and reducing the manufacturing cost.

The damage remaining in the silicon substrate upon ablating a certainthickness of it using a laser is related to the amount of laser energyabsorbed in the substrate that is not used by the ablated material. Ifit can be managed that most of the laser energy is used in removing thematerial then the fraction of the incident energy that seeps into thesilicon substrate is minimized, thus minimizing the laser-inducedsubstrate damage and SRV degradation. The penetration of laser energyinto silicon depends on the laser pulse length (also called pulse width)and wavelength. The infrared (IR) laser beam, wavelength 1.06 microns,has a long penetration depth in silicon, up to about 1000 microns, whilegreen laser beam, with a wavelength of 532 nm, penetrates only to adepth of approximately 3 to 4 microns. The penetration of UV laser beam,with a wavelength of 355 nm, is even shorter, only about 10 nm. It isclear that using ultra-short pulses of UV or EUV wavelength limits thepenetration of the laser energy into silicon. Additionally, shorterlaser pulse length results in shorter diffusion of heat into silicon.While a nanoseconds pulse can lead to heat diffusion in silicon toapproximately 3 to 4 microns range, the picoseconds pulse reduces it toabout 80 to 100 nm, while a femtoseconds pulse is so short thattypically there is no heat diffusion into silicon during the laserablation process. Hence going to shorter pulses with a shorterwavelength lead to diminishing damage to the laser-ablated substrate.For higher production throughput, green or IR wavelengths can be useddepending on the extent of laser damage acceptable. Since even underideal conditions a certain fraction of the energy would still seep intothe substrate, this absorption and its undesirable side effects can befurther reduced by reducing the laser power. However, this results in asmaller thickness of silicon being ablated (or a lower silicon ablationrate or lower throughput). It has been found that reducing the pulseenergy but causing the silicon removal by increasing the overlap of thelaser pulses makes the silicon shallow isolation trench smoother. Thisis an indication of low silicon surface damage. At very low pulseenergies the thickness of silicon removed may be small. The desireddepth may then be obtained by using multiple overlapped scans of thepulsed laser beam.

A pulsed laser beam with pulse length in the picoseconds range and awavelength of approximately 355 nm or below is suitable for siliconablation with low damage enabling low surface recombination velocity(SRV) for passivated ablated surfaces. FIG. 1 shows a 2.25 micron deepand nearly 100 micron wide trench made in a silicon substrate using apicoseconds UV laser beam of Gaussian profile (M²<1.3), nearly 110microns in diameter with 4 microjoule pulse energy, the laser spotsoverlapped nearly 15 times. This depth of ablation was obtained usingtwenty overlapped scans of the laser with each scan removing about 112nm of silicon. FIG. 2 shows the smooth profile of a 4 micron deep and110 micron wide trench in silicon obtained using the same picosecondslaser beam with the UV wavelength. The smoothness of the profile shouldbe noted. Such an ablation of silicon is used in all back-contactback-junction solar cells to form regions that isolate base regions fromemitter regions. Use of a femtoseconds laser may provide furtherreduction of laser damage during silicon ablation.

The embodiments of this disclosure are also applicable to the ablationof amorphous silicon. A similar scheme may be used to ablate a desiredthickness of amorphous silicon using a pulsed laser beam withfemtoseconds pulse length and in some embodiments a UV or greenwavelength. Since ablation of amorphous silicon requires much lowerenergy than crystalline silicon, such a scheme may effectively be usedto selectively ablate amorphous silicon films from the crystallinesilicon surface for application to hetero junction solar cells.

This disclosure is also applicable to oxide ablation selective to theunderlying substrate, crystalline or amorphous silicon. The oxide filmis transparent to laser beams of wavelength down to UV. If a nanosecondspulse length laser is used to remove the overlying oxide, the removal ofoxide takes place by heating and melting of silicon underneath. Becauseof the pressure from the ablated silicon underneath, the overlying oxideis cracked and removed. This however, creates heavy damage in thesilicon substrate so that a wet cleaning treatment is typically used toremove this damaged layer for use in high efficiency cells.

We present here a scheme where the oxide layer is selectively removedfrom the silicon surface without any appreciable damage to the siliconsurface. During the laser ablation, besides heating the material to meltor evaporate it, other effects such as plasma formation take place.Sometimes complex processes can take place at an interface. Using alaser with picoseconds pulse length, the oxide to silicon interface isaffected. Using a picoseconds laser with a UV wavelength, the interfaceeffects are enhanced so that separation and delamination of the oxidefilm takes place from the silicon surface. The silicon surface leftbehind is virtually free of damage. Picoseconds laser radiation withgreen or infra-red (IR) wavelength can also be used depending on howmuch penetration damage of silicon substrate is acceptable. Thisdisclosure will outline the procedure to obtain reduced damage selectiveablation of oxide from the silicon surface.

FIGS. 3A-3D disclose the procedure for obtaining reduced damage ablationof oxide. FIG. 3A shows the variation of laser spot opening in a 1000 APSG (phosphorus-doped oxide)/500 A USG (undoped oxide) stack on a 35micron thick epitaxial silicon film on a template, using a picosecondsUV laser beam. The oxide layers were deposited using APCVD(atmospheric-pressure CVD) technique. For a given thickness of oxide thespot size depends on the laser fluence (J/cm²). The laser fluence is thelaser pulse energy divided by the area of the laser beam. In this case,the laser beam was about 100 microns in diameter with a Gaussian profile(M²<1.3). At very low fluence, the spots are irregular and there isirregular delamination of oxide from the silicon surface as shown inFIG. 3B, while at very high fluence there is extensive damage of siliconas shown in FIG. 3D. The range of fluence shown by line a-a′ indicatesthe optimum range where the damage to the silicon substrate is minimalas seen in FIG. 3C.

FIG. 4 shows rows of cell contact openings that are selectively openedin the oxide for application in all back-contact (and back-junction)solar cells. FIG. 5 is a close-up of these contacts. The laser ablationspots can be overlapped in both x and y direction to open up an area ofany desired length and width on the wafer as shown in FIGS. 6A and 6B.FIG. 6A shows a 180 micron wide opening made by selectively removing theBSG (boron-doped oxide) for base isolation region using picoseconds UVlaser beam with ablation spots overlapping in both x and y-direction.Similarly, FIG. 6B shows a 90 micron wide area opened up in USG (undopedoxide) for forming the base region.

The selective ablation of oxide from a silicon surface as disclosed herecan be used in solar cell making in several ways. In one application,when using in-situ emitter for back-contact cells, this process is usedto open tracks in an oxide film to expose the underlying emitter. Theemitter so exposed may be removed using wet etching. This region is thenused for base to emitter isolation and with base formed inside it.

In another application, this process is used to open regions that arethen used for making metal contacts. For front contacted cells, theoxide passivation can be used on the backside of the cells. The schemedescribed here is then used to open contacts for the metal that issubsequently deposited on these contacts. In this manner, the metal haslocalized contact that is conducive to high cell efficiency. For backcontacted cells, contacts for both base and emitter may be opened usingthis scheme.

In a solar cell process flow, a doped oxide may need to be removedwithout causing any doping of the silicon underneath (i.e., without anyappreciable heating of the doped oxide and silicon structure). Since, asdescribed above, the oxide is removed by separation at the oxide/siliconsubstrate interface when using a picoseconds laser beam, the removal ofoxide happens with limited pickup of the dopant from the oxide filmbeing ablated.

The selective ablation of silicon nitride (SiNx) is used for frontcontacted solar cells. Using laser ablation, the contact area to theemitter surface can be reduced thereby minimizing the area where the SiNpassivation is removed. This leads to higher Voc. Picosecond lasers witheither UV or green wavelength are suitable for this application,although nanoseconds UV lasers can also be used.

Selective metal ablation from the oxide surface has historically beendifficult using lasers. This is because at the high pulse energiesneeded to ablate metal, the energy is high enough to damage the oxideunderneath and cause penetration of metal into oxide. In fact, this isthe basis for the process of “laser fired contacts” (LFC) proposed foruse in the solar cells.

We disclose three schemes for selectively removing metal from the oxide(or another dielectric) surface with no metal penetration of oxide (orother dielectrics such as silicon nitride) and breaking or cracking ofoxide. In all these schemes, aluminum is the first metal in contact withbase and emitter (aluminum being used as the contact and light trappingrear mirror layer). A laser with picoseconds pulse length is suitablefor this application. For high metal removal rate the IR wavelength isquite suitable. According to the first scheme, metal is ablated at apulse energy that is lower than the threshold for oxide ablation. If thethickness of metal removed in one scan is lower than the desiredthickness, multiple overlapping scans are used to remove the fullthickness of metal. Since the pulse energy is below the oxide ablationthreshold, a clean removal of metal from the oxide surface is obtained.However, the exact recipe used highly depends on the type of metal inthe stack, their thickness and surface roughness, etc.

FIGS. 7A-7C shows the ablation results when patterning a PVD-depositedbi-layer stack of 2400 A of NiV on 1200 A of Al on oxide. It is desiredthat the metal be removed completely between the runners withoutbreaking through the oxide layer underneath (to prevent shunts in thecell). FIG. 7A shows the threshold for pulse energy, below which thismetal stack can be removed without penetration of oxide. This threshold,besides depending on the metal stack characteristics described above,depends on the laser parameters such as spot overlap obtained using acertain pulse repetition rate of the laser as well as the scan speed.With increasing pulse overlap the threshold pulse energy would decrease,because of the energy accumulation in the metal. FIG. 7B shows thatusing a pulse energy below the threshold for oxide damage, more thantwenty scans provided complete isolation of metal runners as determinedby the 100M-ohm resistance between parallel lines. FIG. 7C shows a clean75 micron trench formed in the 2400 A NiV/1200 Al metal stack.

According to the second, high-throughput scheme higher pulse energiesare used, since a substantial part of the incident energy is absorbed asit is being ablated thereby reducing damage to the oxide. This approachmakes the laser ablation of metal a very high throughput process. Usingthis scheme we have ablated 1250 A Al/100-250 A of NiV, with or withouta tin (Sn) overlayer up to a thickness of 2500 A successfully using atwo step process. In the first step the softer metal is removed using 15microjoule pulses, followed by 30 microjoule pulses both overlappedfifteen times. For thicker aluminum such as 2000 A the second step canbe carried out at 50 microjoules with the same number of overlapping ofpulses.

The third scheme of metal ablation is applicable to highly reflectivefilms, for example Al/Ag stack (with Al in contact with the cell and Agon top of Al), such that most of the incident energy of the picosecondslaser is reflected and ablation is drastically reduced. In that case thesurface of the reflective metal (Ag) is first dented using a long pulselength nanoseconds laser, pulse length from 10 to 800 nanoseconds,followed by picoseconds cleanup of the aluminum underneath.

This disclosure is also applicable to the selective doping of asubstrate. For successful doping of silicon using an overlying layer ofthe dopant-containing material, the pulse energy should be high enoughto melt the silicon but not high enough to ablate it or the dopant layerabove it. As the silicon melts, the dopant is dissolved into it. Uponrecrystallization of this silicon layer, a doped layer is obtained. Forthis application a nanoseconds pulse length laser with green wavelengthis quite suitable because of its limited penetration into silicon.

The laser processing techniques described above are applicable to planarand 3-D thin-film crystalline silicon substrates. The laser processesdescribed here are suitable for any thickness of the silicon substrate.These include the current standard wafer thickness of >150 microns usedfor crystalline silicon solar cells. However, they become even moreadvantageous for thin, fragile wafers or substrates as the process incarried out without any contact with the substrate. These include thewafers thinner than 150 micron obtained from monocrystalline CZ ingotsor multi-crystalline bricks using advanced wire sawing techniques or byother techniques such as hydrogen implantation followed by annealing toseparate the desired thickness of wafer, or thin-film monocrystallinesubstrates (such as in the thickness range of from a few microns up to80 microns) obtained using epitaxial deposition of silicon on asacrificial separation/release layer such as porous silicon and itssubsequent lift off.

The laser processing is uniquely suited to three dimensional substratesobtained using pre-structuring of reusable templates and siliconmicromachining techniques. One such method is described in the '713Application. FIGS. 8A through 9B show the 3-D thin film siliconsubstrates obtained using the technique described in that publication.FIG. 8A shows the top view while FIG. 8B shows the cross-section of theTFSS so obtained. For pyramidal substrates, the tips may be flat or mayend in a sharp point. FIGS. 9A and 9B show the TFSS with prism structureobtained using a reusable pre-structured 3D template described in thereference above.

Although the laser processes and the process flows described here areapplicable to any thickness of the silicon substrate (from less than onemicron to over 100 microns), we disclose here their application to solarcells made using thin silicon substrates in the thickness range of fromless than 1 micron to about 80 microns, including but not limited tothose that are obtained using epitaxial silicon on porous siliconsurface of a reusable template as described in the '713 Application. Tofacilitate the understanding of our application, the process flow forobtaining a desired thickness (e.g. from about less than 10 microns upto about 120 microns) of planar monocrystalline TFSSs according to thatpublication is shown in FIGS. 10A and 10B for planar TFSS that aretypically greater than about 50 microns so that they can be handled asself supporting substrates during cell processing, and FIGS. 11A and 11Bfor planar TFSS that is typically thinner than about 50 microns so thatthey are not self supporting during cell processing (and hence, arereinforced prior to separation from their host templates). FIGS. 12A-12Dshow the process flow for obtaining three-dimensional pyramidal siliconsubstrates. Three-dimensional prism-shaped substrates can be obtainedwith similar processes, but using a lithography or screen printedpattern that provides for that structure.

The thin planar substrate obtained using the process flow of FIGS. 10Aand 10B may be processed according to the process flow of FIG. 13 toobtain high efficiency front contacted solar cells. It should be notedfor self-supporting TFSSs it is advantageous to process the templateside of the TFSS first before proceeding to the other side. Since thetemplate side of the TFSS is textured during the removal of thequasi-monocrystalline silicon remaining on the TFSS after its separationfrom the template it is preferably the frontside or sunnyside of thesolar cell. The laser processes of selective ablation of silicon oxideand silicon nitride (SiN) are used to advantage in making this frontcontacted solar cell.

FIG. 14 shows the application of various laser processes for making highefficiency front contacted solar cells using planar TFSSs where the TFSSis too thin to be free standing or self supporting during cellprocessing. It should be noted that in this case the non-template sidesurface is processed first with the TFSS on the template. Once thisprocessing is complete the TFSS is first attached to a reinforcementplate or sheet (also called a backplane) on the exposed processed sideand then separated from the template. After separation of thebackplane-attached (or backplane-laminated) thin-film crystallinesilicon solar cell, removal of residual porous silicon, texture etch,and SiN passivation/ARC deposition, and forming-gas anneal (FGA)operation processes are carried out on the released face of TFSS (whichwill end up being the front surface of the solar cell).

FIG. 15 shows the application of various laser processes for making highefficiency front contacted solar cells using 3-D front TFSS. For thisapplication it is advantageous to have pyramid tips on the template sidenot be sharp but end in flat ledges.

The processes described here are further uniquely suited to simplifyingthe all back-contact cell process flow.

FIGS. 16A-16D show the laser processes used on the planar epitaxialsubstrate to make a back-contact/back-junction solar cell where the TFSSis self supporting (i.e., no backplane attachment to the cell). In thisapplication the epitaxial emitter is deposited in-situ during siliconepitaxy following the deposition of the epitaxial silicon base. Theablation of silicon is then used to remove the emitter from the baseisolation regions. At the same time four fiducials are etched into oxideto align subsequent ablation to this pattern. Next, a thermal oxide isgrown to passivate the silicon surface that will become the back surfaceof the back-contact back-junction solar cell. The epitaxial silicon filmis then disconnected or released from the template (by mechanicalrelease from the porous silicon interface). Next, the residual poroussilicon layer is wet etched and the surface is textured (both can bedone using an alkaline etch process). This will become the texturedfront surface or the sunnyside of the solar cell. Now, the thermal oxideis ablated using a picoseconds UV laser to form base openings inside thebase isolation region. The base opening is aligned inside the baseisolation region (trench) formed by silicon ablation earlier using thefiducials that were etched in silicon earlier as mentioned above. Next aphosphorus containing oxide layer (PSG) is blanket deposited on thesurface. Scanning with a nanosecond green or IR laser aligned to baseopening using the fiducials in silicon causes the base to be doped.Also, the region that will have the contact openings to emitter is alsodoped in a similar manner using the aligned scans of nanosecond green orIR laser. Next, contact opening are made to these doped base and emitterareas using a picoseconds UV laser. Again, the alignment of thesecontact openings is made using fiducials in silicon. Now, a metal stacklayer comprising aluminum as its first layer in contact with the cell(e.g., a stack of 1250 A Al/100-250 A NiV/2250 Sn) is deposited using asuitable method such as a PVD (physical vapor deposition) technique.Next, this layer is patterned using a picoseconds IR laser so that themetal runners are separately connected to the base and emitter regions.After an optional forming gas anneal (FGA), the cell is connected to andreinforced with a backplane with either embedded (Al or Cu)high-conductivity interconnects or no embedded interconnects (in thelatter, the final cell metallization can be formed by a copper platingprocess). The cell is now ready for test and use.

FIG. 17 shows the laser processes used on the planar epitaxial substrateto make a back-contact solar cell where epitaxial silicon base is notdeposited with an emitter layer. Instead, a boron containing oxide (BSG)layer is deposited and patterned to open the base isolation region. Asimilar process to that described above is followed except that now theemitter and base are formed simultaneously during a thermal oxidationstep according to the process flow outlined in FIG. 17.

FIG. 18 shows a process flow using laser processes on the epitaxialsubstrate to make a planar back-contact/back-junction solar cell wherethe TFSS is not self-supporting (hence, a backplane is used). This flowuses the silicon ablation of in-situ doped emitter to form the baseisolation region.

FIG. 19A-19H show a process flow using laser processes on the epitaxialsubstrate to make a planar back contact solar cell where the TFSS is notself-supporting. In this flow, instead of an in-situ emitter layer, theBSG deposition and selective laser ablation followed by thermaloxidation (or a thermal anneal or a thermal oxidizing anneal) is used toform the emitter as well as the base isolation region.

FIG. 20 shows a process flow for making back contacted 3-D solar cells,it is advantageous to have the template side of pyramids end inrelatively sharp points. Since the 3-D TFSS can be self-supporting torelatively low thickness (e.g., silicon as thin as about 25 microns),the process flow is similar to that shown in FIG. 16. It should be clearthat we again have a choice of using the in-situ emitter followed bylaser ablation of silicon, or BSG deposition and selective laserablation followed by thermal oxidation (or thermal anneal, or thermaloxidizing anneal).

For applications in hetero-junction solar cells, a hetero-junctionemitter may be formed by a doped amorphous silicon layer in contact withan oppositely doped crystalline silicon base. For interdigitated backcontact solar cells we pattern the amorphous silicon layer and thetransparent conducting oxide (TCO) using laser ablation that isselective to the crystalline layer. Femtoseconds pulsewidth lasers witheither UV or green wavelength are suitable for this application. Aprocess flow is described in FIG. 21. Several variations of this 30process flow are possible.

Various embodiments and methods of this disclosure include at least thefollowing aspects: the process to obtain ablation of crystalline andamorphous silicon with reduced damage; the process to obtain oxideablation for both doped and undoped oxides with reduced damage tosilicon; the process to obtain fully isolated metal patterns on adielectric surface for solar cell metallization; the process toselectively dope the emitter and base contact regions; the use of pulsedlaser processing on very thin wafers, including planar and 3-D siliconsubstrate; the use of pulsed laser processing on substrates obtainedusing epitaxial deposition on a reusable template made using templatepre-structuring techniques; the use of various pulsed laser processes inmaking front contacted homo-junction solar cells; the use of variouspulsed laser processes in making all-back contacted homo-junction solarcells; and the use of various pulsed laser processes in makinghetero-junction solar cells.

Although the front contact solar cells are described with p-type baseand back-contact back-junction solar cells are described with n-typebase, the laser processes described here are equally suited to thesubstrate with opposite doping, i.e., n-type for front contact solarcell with P³⁺ emitter, and p-type base for back-contact back-junctionsolar cells with p-type base and n⁺ emitter.

The following description, tables, and figures disclose the applicationof flat top laser beams to laser processing methods for interdigitatedback-contact cells (IBC). FIGS. 22A through 30 are not found in U.S.patent application Ser. No. 13/118,295 “LASER PROCESSING FORHIGH-EFFICIENCY THIN CRYSTALLINE SILICON SOLAR CELL FABRICATION” byVirendra V. Rana and filed on May 27, 2011. The description following isdirected towards methods for the formation of back contact solar cellsutilizing flat top laser beams as compared to traditional Gaussian laserbeams. Further, the implementation of flat top laser beams to the laserprocessing methods described throughout this application providessubstantial reduction in damage to silicon, improvement in solar cellfabrication throughput, and a bigger alignment window for definingpatterns (e.g. patterns of emitter and base regions) that are insetinside another pattern.

FIGS. 22A and 22B are schematics showing the profile of a Gaussian beam,FIG. 22A, and a flat top beam, FIG. 22B. The beam intensity of theGaussian beam has a smooth decrease from a maximum at the beam center tothe outside of the beam. In contrast, the intensity is “flat” or uniformfor the flat top beam through most of its profile (center to outside).

As disclosed herein, high-efficiency back-contacted, back-junction cellswith interdigitated back contact (IBC) metallization benefits from theuse of at least one or several steps of pulsed laser processing. Laserprocessing may be utilized in several processing steps throughout theformation of the back contact cell, including: delineating/definingemitter and base regions (or base-to-emitter isolation region), definingback-surface field (BSF) or base regions, doping to form back surfacefields (by laser irradiation), selective doping of contacts, openingcontacts in the dielectric to base and emitter, and metal patterning.Some of these steps require laser processing of wide areas that aretypically produced by overlapping Gaussian beam laser spots. Overlappingseverely reduces cell processing speed and may cause silicon damage,resulting in degradation of cell performance and yield. By using flattop beam methods disclosed herein, the overlapping of spots isdramatically reduced so that the semiconductor (e.g., crystallinesilicon) substrate damage is significantly reduced and throughput isincreased. Also, smaller diameter Gaussian spots may be replaced with arelatively wide flat top laser beam which may further substantiallyincrease the throughput.

FIGS. 23-25 illustrate embodiments of back contact solar cells that maybe formed according to the disclosed flat top laser beam processingmethods.

FIG. 23 is a cross-sectional diagram of a back-contact/back-junctioncell with interdigitated back-contact (IBC) metallization formed from ann-type substrate, such as that disclosed herein. As shown in FIG. 23,alternating emitter and base regions are separated by relatively lightlyn-doped substrate regions (the n-type base). The rear/backside surfaceis covered by a surface passivation layer that provides good surfacepassivation with low back surface recombination velocity, made of, forexample: thermal silicon dioxide, deposited silicon dioxide, or siliconoxide/silicon nitride layers which may be deposited using techniquessuch as PECVD or APCVD (and/or aluminum oxide deposited by atomic layerdeposition or ALD). This surface passivation process may then befollowed by making openings in this passivation layer which act as‘localized contacts’ to the emitter and base regions. Then conductordeposition and patterning (e.g., aluminum as shown in FIG. 23) may beperformed to separately connect the emitter and base regions.

FIG. 24A is a rear/backside view of a back contact solar cellillustrating an interdigitated back contact base and emitter design withthe emitter and base regions laid out in alternating parallel rows. Thisbackside may be formed, for example, by starting with a surface that iscompletely covered by an emitter region, then delineating a base regionresulting in the formation of the patterned emitter regions. Then dopingbase contact regions with phosphorus is carried out and contacts areopened to the base and emitter regions in preparation for metallization.

FIGS. 24B-24F are rear/backside views of a back contact solar cellillustrating the back contact cell after key processing steps, whereinany one step or combination of steps may be performed according to alaser process which may or may not utilize a flat top beam. The variouslaser patterning steps of this particular exemplary method are outlinedin FIGS. 24B-24E. Starting with an n-type silicon substrate, a BSG layeris deposited over the whole surface. Next, the emitter to BSF isolationregion is defined using laser ablation of the BSG as shown in FIG. 24B.This step, the delineation of base and emitter regions, is referred toherein as the “BSG Opening” step. Alternatively, an in-situ boron dopedlayer may be deposited during silicon epitaxy and the BSF region definedusing laser ablation of silicon.

After the emitter to BSF isolation region is defined in the BSG Openstep, a USG layer is deposited on the wafer followed by laser ablationof this layer in patterns that are inlaid to the BSG Open region, asshown in FIG. 24C. This patterning step is referred to herein as the BSFOpening step or base opening step. The BSF openings should be isolatedfrom the edges of the BSG Openings to prevent shunt formation as shuntsare deleterious to the solar cell efficiency.

Next, a PSG layer is deposited on the wafer and the silicon exposed toPSG in the BSF opening is doped using selective laser scans of thisarea. The doped BSF regions (base regions) are outlined in FIG. 24D

Next, the contacts to base and emitter are made using laser ablation asshown in FIG. 24E. It should be noted that the contacts may be pointcontacts as shown in FIG. 24E or line contacts as shown in FIG. 24F.Also, the number of contacts or the number of lines should be optimizedfor minimum series resistance of the current conduction path for thesolar cell—thus the designs and methods of the disclosed subject matterare not limited to the exemplary embodiments shown herein. It is alsoimportant that the contact openings are properly aligned inside theparticular doped area so that there is no current leakage.

As disclosed previously, a picoseconds pulse length laser may be usedfor oxide ablation processes of BSG open, BSF opening, and contactopening, although a nanoseconds pulse length laser may also be used.Further, although IR wavelength may be used, green or UV or smallerwavelengths are more suitable because of their reduced penetration intosilicon.

For BSF doping particularly, a nanoseconds pulse length laser may bemore suitable because of its penetration into silicon. And although IRwavelength may be used, green wavelength, because of its reducedpenetration compared to IR, may be more suitable for the depth of dopingtypically desired.

FIG. 25 is a rear/backside view of the back contact solar cell of FIG.24A with alternating metal lines contacting the emitter and baseregions. Note that the metal lines for the emitter and base regions areseparately connected to busbars not shown in FIG. 25 for simplicity ofthe figure. This metal pattern may be formed by blanket deposition of ametal followed by laser ablation of the metal to isolate base contactsfrom emitter contacts. Because relatively thick metal lines are requiredfor good current conduction (usually lines 20 μm thick or thicker), athinner metal stack such as aluminum/nickel-vanadium/Tin may be firstdeposited and patterned by lasers, followed by the selective depositionof a thicker metal such as copper using electro or electroless plating.Alternatively, a backplane with relatively thick conductors may beapplied and attached to the cell with thin conductor lines. Apicoseconds pulse length laser with IR wavelength may be most suitablefor ablating the metal stack with good selectivity to the underlyingoxide layer.

The disclosed flat top laser beam processing steps that may be utilizedto make this structure possible include, but are not limited to:delineation of emitter and base regions (BSF and emitter to BSFisolation) by laser ablation of an emitter or deposited boron dopingdielectric (such as boro-silicate glass BSG deposited by APCVD);delineation of the BSF region by opening the dielectric covering theopening made in the BSG; N+ doping of the base (e.g., with phosphorus);opening of metallization contacts to base and emitter regions; and metalpatterning using metal laser ablation to isolate base and emittercontacts. FIGS. 26A-26C are diagrams illustrating three ways a flat-topbeam profile may be created (diagrams reproduced from F. M. Dickey andS. C. Holswade, “Laser Beam Shaping: Theory and Techniques”, MercelDekker Inc., NY, which is hereby incorporated by reference in itsentirety). FIG. 26A illustrates one technique for creating a flat topbeam profile, the so -called “aperturing of the beam.” Using thismethod, the Gaussian beam is made flatter by expanding it and anaperture is used to select a reasonable flat portion of the beam and tocut-out the gradually decreasing ‘sidewall’ areas of the beam. Usingthis method, however, may cause a significant loss of beam power.

A second example method for creating a flat top beam, as shown FIG. 26B,uses beam integration wherein multiple-aperture optical elements, suchas a micro-lens array, break the beam into many smaller beams andrecombine them at a fixed plane. This beam integration method may workvery well with beams of high M² value.

A third beam shaping system for creating a flat top beam, as shown FIG.26C, uses a diffractive grating or a refractive lens to redistribute theenergy and map it to the output plane. Any known method, including thethree example techniques disclosed in FIGS. 26A-26C, may be used obtainthe flat top beam profile for applications described herein. Thesuitability and choice of a flat top laser beam formation method dependson a variety of factors including the available beam characteristics andthe results desired.

FIGS. 27A and 27B are schematics showing the profile of a Gaussian beamand a flat top beam highlighting the ablation threshold. As shown inFIGS. 27A and 27B, a flat top laser beam, particularly as compared to aGaussian beam, can substantially reduce the laser damage during ablationand doping processing. For Gaussian beams there is substantial excessivelaser intensity above that required for ablation, particularly in thecenter of the beam, that can cause damage of silicon (as shown in FIG.27A). The flat top beam can be configured so the peak intensity is onlyslightly above that required to ablate the material (the ablationthreshold as shown in FIG. 27B) and the damage that may be caused by thehigh intensity of the Gaussian beam is avoided.

A flat top beam, whether having a square or rectangular cross section,offers throughput advantages particularly as compared to a Gaussianbeam. FIG. 28A is diagram showing a Gaussian beam ablated regionprofile/footprint. The circular shaped spots of a Gaussian beam arerequired to overlap substantially to the minimize the zigzag outline ofthe pattern, typically as much as 50% overlap (FIG. 28A). FIG. 28B isdiagram showing a flat top beam ablate region profile/footprint. Sincethe square or rectangular flat top beam have flat edges, thus creating aflat outline, the overlap can be significantly reduced (FIG. 28B). FIG.28C is a graph showing the improvement in scan speed as beam overlap isreduced. Note that even for an overlap of 30%, a scan speed increase of33% may be realized.

FIG. 29A is a diagram illustrating a beam alignment window of a Gaussianbeam and FIG. 29B is a diagram illustrating a beam alignment window of aflat top beam. As can be seen in FIGS. 29A and 29B, yet anotheradvantage of using a flat top beam for making inlaid patterns is thelarger alignment window the flap top beam provides. The circular shapedspots obtained from a Gaussian beam create zigzag edges of the ablatedregions (FIG. 29A). The alignment margin of M as shown in FIG. 29A isreduced and limited to M-a-b due to the waviness of the zigzag edgeprofile.

However, the ablation region edges created using a flat top beam arestraight allowing the alignment margin to stay at M. For the backcontact back junction solar cells described herein, BSF openings areformed inside the BSG Open regions, and contact openings are formedinside the BSF region. Hence, a larger alignment margin is important asit allows for smaller BGS Open, BSF, and contact regions. Thus reducingthe electrical shading and improving solar cell performance.

Since the overlap of square or rectangular flat top beam can be reducedin both x and y direction while making a large area ablation or doping,the throughput is significantly enhanced. Also, since the size of thesquare or rectangular flat top can be increased without causingexcessive zigzagging of the perimeter, throughput is further increased.Table 1 shows the reduction in the number of scans needed to open a 150um wide line, such as used for delineating the base area by ablating theBSG film.

Table 1 below shows the throughput of Gaussian vs. Flat Top laser beamsfor creating a 90 μm wide base opening. The results of Table 1 are showngraphically in FIG. 30C.

TABLE 1 Width Spot Pitch of Number of line Size scans of scans PROCESS(um) (um) Overlap % (um) per line BSG Ablation with 150 30 50 15 9Gaussian BSG Ablation with Flat 150 30 20 24 6 Top BSG Ablation withFlat 150 60 20 48 3 Top

FIG. 30C shows the throughput advantage of flat top beams (the 60 μmflat top beam region profile is depicted in FIG. 30B) as compared to theGaussian beam (the 30 μm flat top beam region profile is depicted inFIG. 30A), for a high productivity laser system that can process fourwafers at a time. To further reduce cost, for example, two lasers may beutilized with each laser beam further split into two. However, manyvariations of this flat top laser beam hardware and fabrication schemeare possible.

Also, because overlap is significantly reduced in both x and ydirections when using a flat top beam, the laser induced damage ofsilicon is greatly reduced as compared to the Gaussian beam.

Similar throughput advantages may also result when utilizing a flat topbeam for opening the oxide region for BSF, doping the BSF region usingthe overlying PSG, forming baseand metal contact openings if they areline contacts, and the metal ablation isolation lines—all with theconcurrent advantage of reduced silicon damage. Additionally, utilizinga flat top beam provides the advantage of increased alignment window forBSF opening inside the BSG opening and contact opening inside the BSF.Flat top laser processing methods may also increase throughput forforming a back surface field. For example, the back surface field may beformed by doping the base region, opened as described, with an n-typedopant such as phosphorus. For this process the base is covered with aphosphorus-doped silicon oxide (PSG) layer and the doping may beperformed by irradiating this region with a laser beam. While uniformlydoping this region using Gaussian laser beams requires overlapping,overlapping is minimized or may be completely reduced using a flat topbeam. And as with the base and emitter region delineation and backsurface field delineation described herein, utilizing a flat top laserbeam provides a substantial throughput and reduced damage advantage asrequired overlapping is decreased. It should be noted that for forming aback surface field, the beam need to be flat top beam only in onedirection—normal to the scan, whereas it may be Gaussian in thedirection of the scan. This type of beam is called a hybrid flat topbeam.

Importantly, for forming isolated base or emitter contacts, althoughoverlap is not an issue, the silicon damage is still reduced using aflat top beam because of the absence, unlike Gaussian, of a highintensity peak in the center of the beam (as shown in FIGS. 27A and27B).

Those with ordinary skill in the art will recognize that the disclosedembodiments have relevance to a wide variety of areas in addition tothose specific examples described above.

The foregoing description of the exemplary embodiments is provided toenable any person skilled in the art to make or use the claimed subjectmatter. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without the use of theinnovative faculty. Thus, the claimed subject matter is not intended tobe limited to the embodiments shown herein but is to be accorded thewidest scope consistent with the principles and novel features disclosedherein.

It is intended that all such additional systems, methods, features, andadvantages that are included within this description be within the scopeof the claims.

What is claimed is:
 1. A method of processing a thin crystalline siliconsubstrate, said method comprising the steps of: delineating emitter andbase regions with pulsed laser ablation utilizing a flat top laser beamon a thin crystalline silicon substrate making openings defining emitterto base isolation regions, said substrate having a thickness in therange of approximately 1 micron to 100 microns suitable for use in aback-contact back-junction solar cell; performing pulsed laser ablationwith a flat top laser beam on said thin silicon substrate to form baseopenings; irradiating said base region with a flat top laser beam toform doped base regions; selectively doping said emitter regions;performing pulsed laser ablation with a flat top laser beam to opencontacts for base regions and emitter regions; forming metallization onsaid base regions and said emitter regions; and performing pulsed laserablation with a flat top laser beam of said metallization to form aninterdigitated pattern of metal lines connected to said base regions andmetal lines connected to said emitter regions.
 2. The method of claim 1,wherein said flat top laser beam is created according to an aperturingof the beam method.
 3. The method of claim 1, wherein said flat toplaser beam is created according to a beam integration method.
 4. Themethod of claim 1, wherein said flat top laser beam is created accordingto a diffractive grating method.
 5. The method of claim 1, wherein saidstep of delineating emitter and base regions with pulsed laser ablationutilizing a flat top laser beam on a thin crystalline silicon substratefurther comprises delineating said emitter and said base regions in aninterdigitated pattern.
 6. The method of claim 1, wherein said step ofperforming pulsed laser ablation with a flat top laser beam of saidmetallization is carried out below an oxide ablation threshold for saidthin crystalline silicon substrate.
 7. The method of claim 1,delineating emitter and base regions with pulsed laser ablationutilizing a flat top laser beam on a thin crystalline silicon substrateis carried out via pulsed laser ablation of a deposited borosilicateglass layer.
 8. The method of claim 1, wherein said step of irradiatingsaid base region with a flat top laser beam to form doped base regionsfurther comprises irradiating a phosphorus-doped silicon oxide layer toform doped base regions.
 9. The method of claim 1, wherein said step ofdelineating emitter and base regions with pulsed laser ablationutilizing a flat top laser beam is carried out via a laser ablationprocess and using a pulsed laser having a wavelength of approximately800 nm or less and a pulse width less than approximately 100picoseconds.
 10. The method of claim 9, wherein said wavelength isapproximately 355 nm or less.
 11. The method of claim 9, wherein saidpulse width is less than approximately 20 picoseconds.
 12. The method ofclaim 1, wherein said irradiating said base region with a flat top laserbeam to form doped base regions further comprises irradiating said baseregion with a hybrid flat top laser beam to form doped base regionsfurther.
 13. A method of processing a thin crystalline siliconsubstrate, said method comprising the steps of: delineating emitter andbase regions with pulsed laser ablation utilizing a flat top laser beamon a thin crystalline silicon substrate, said substrate having athickness in the range of approximately 1 micron to 100 microns suitablefor use in a back-contact back-junction solar cell; performing pulsedlaser ablation with a flat top laser beam on said thin silicon substrateto form base openings; irradiating said base region with a hybrid flattop laser beam to form doped base regions; selectively doping saidemitter regions; forming isolated contacts for base regions and emitterregions.